This invention relates to a connecting arrangement with a threaded sleeve or coupling of synthetic plastic material, preferably of a polyolefin such as for example polyethylene, and with a tube portion having a pipe thread, particularly a metal pipe or metal fitting, in which the threaded coupling comprises an injection molded insert nut member with internal threads and an inner synthetic plastic ring engaged by the pipe thread of the tube portion.
German Published Application DE-OS No. 33 15 030 discloses a threaded coupling of synthetic plastic material for connection to a metal pipe or metal fitting having a pipe thread. In order to connect the metal pipe to the threaded coupling with a tight seal and still have the parts releasable from each other, this arrangement comprises a metal insert nut which is provided in its forward region with an internal thread extending through a prescribed length. Adjoining the internal thread is a jacket having an internal diameter greater than the outer diameter of the internal thread. The interior of the jacket is filled with a synthetic plastic ring of the threaded coupling. When the metal pipe or metal fitting is screwed into the coupling, its pipe threads cut into or are pressed into the synthetic plastic ring. In installations which must meet high safety requirements, particularly in gas lines, or also in installations subjected to high pressures, the degree of seal in this known connecting arrangement may not be sufficiently great. Thus, particularly in view of applications requiring long service lives of many years or decades, difficulties may arise with the respect to the functional reliability. German Patent DE-PS No. 12 23 208 discloses a pipe connection for synthetic plastic pipes in which both pipe ends have an external thread and are coupled to each other with a threaded connector. The threaded connector contains at one end at least one nose extending parallel to the axis and having a contact surface or stop. One pipe end has a radial nose associated with the contact surface. This ensures that the threaded connector is not screwed too far, but also not to little, onto the one pipe end. A transition from a metal pipe to a synthetic plastic pipe is not necessarily readily achievable with this arrangement, and no really high demands can be made on the tightness of the seal.
French Patent Application FR No. 2 452 657 discloses an arrangement in which a synthetic plastic pipe is connected with a metal nut. A connecting piece is introduced into the end of the synthetic plastic pipe which is anchored in an undercut in the interior of the nut. The exterior surface of the connecting piece is provided with teeth and grooves. The end of the pipe and the connecting piece are surrounded by an injection molded piece which is injection molded around these two parts after the intermediate piece is introduced into the end of the pipe. Suitable tools and machinery are required to produce this injection molded piece, i.e. for molding around the end of the synthetic plastic pipe. Sealing of the connecting piece with respect to the threaded nut is achieved by screwing the threaded nut onto a threaded support which requires further sealing elements.
British patent application GB No. 21 33 850 discloses a connecting arrangement in which a first part is slid into a synthetic plastic pipe. This first part is provided on its outer surface with a radial rib, and the synthetic plastic pipe is pressed onto this rib by means of a threaded nut surrounding the outside of the pipe. The radial rib has small ribs or teeth on its outer surface which are pressed into the inner surface of the synthetic plastic pipe. Sealing takes place only in the vicinity of the aforementioned rib, and a supplemental securing means or seal is not provided.